There is a need for three-color flat panel displays such as liquid crystal displays ("LCDs"). The most common way to fabricate an LCD having red, green, and blue colors is to use color filters positioned in optical series with the LCD. The display elements or "pixels" of the LCD itself are designed to be achromatic in that they display only shades of neutral gray. Color is obtained by spatially aligning to the pixels a filter comprised of repetitive groups of adjacent red, green, and blue stripes, each having the width of one pixel. The display electronics sends red, green, and blue image information to corresponding pixels aligned with the respective red, green, and blue stripes. The result is a red, green, and blue pixel for every pixel of full color information.
The prior art has two basic kinds of color filters for LCDs. These include dyed filters and vacuum deposited interference filters. Dyed filters are made of colored dyes that are impregnated into a particular type of polymer matrix. Dyed filters are difficult to make, requiring separate patterning and dying operations for all three colors. Moreover, dyes are limited by gradual color transition edges and low transmission properties. The gradual color transition edges permit the other colors to leak through and thereby degrade color saturation, and the low color transmissions degrade the brightness of the LCD image.
The transmission spectra of red, green, and blue colored dye filters are superimposed in FIG. 1. With reference to FIG. 1, each of the red transmission spectrum 12, green transmission spectrum 14, and blue transmission spectrum 16 is broad and not highly transmissive. The broadness of the spectra degrades the color saturation as a consequence of light leakage from adjacent spectral colors. The dyes do, however, have the advantage of not appreciably changing colors with angle of view because the absorption properties of the dyes are angle independent. For off-axis angles, the transmission decreases slightly because of an increased path length through the dye.
The vacuum deposited interference filters are constructed with thin film layers evaporated in a vacuum chamber. Although they have high transmission properties and rapid color edge transitions, interference color filters have never become commercially accepted because of the high cost associated with their construction. The reason is that, for each of the three colors, a filter must be patterned with photoresist, placed in an evacuated coating chamber, and have properly aligned film layers. Along with the high cost, vacuum deposited interference filters change their colors with angle because the apparent thickness of the interference layer changes with angle. The three spectra of such a vacuum deposited filter are shown in FIG. 2. FIG. 2 shows similar curves for filters made by vacuum deposited thin films that use optical interference to produce colors. With reference to FIG. 2, red transmission spectrum 18, green transmission spectrum 20, and blue transmission spectrum 22, have much sharper edge transitions than those of FIG. 1, produce more saturated colors, and have higher transmissions. Interference filters are made of discrete layers of high and low index materials, instead of the ideal sinusoidal variations, so there is usually some undesirable "ringing" of the spectra.
A holographic filter is a wavelength selective type of color filter that is sometimes called a "holographic mirror." Holographic mirrors have been produced for avionics head-up displays and eye protection shields from laser light. FIG. 3 shows an exemplary spectrum 24 of a holographic filter. No prior art filter includes segmented holographic mirrors arranged in arrays of color filters.